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This uses therubyracer to bind to the actual JavaScript implementation of Handlebars.js so that you can use it from ruby.


Simple stuff

require 'handlebars'
handlebars = Handlebars::Context.new
template = handlebars.compile("{{say}} {{what}}")
template.call(:say => "Hey", :what => "Yuh!") #=> "Hey Yuh!"

functions as properties

template.call(:say => "Hey ", :what => lambda {|this| ("yo" * 2) + "!"}) #=> "Hey yoyo!"

Block Helpers:

Just like JavaScript, you can write block helpers with an {{else}} section. To print out a section twice if a condition is met:

# V8 maps the first argument sent to a block to "this". All subsequent arguments are as
# described in the Handlebars documentation.
handlebars.register_helper(:twice) do |context, condition, block|
  if condition
template = handlebars.compile("{{#twice foo}}Hurray!{{else}}Boo!{{/twice}}")
template.call(foo: true) #=> Hurray!Hurray!
template.call(foo: false) #=> Boo!

Private variables:

Just like JavaScript, block helpers can inject private variables into their child templates. These can be accessed in a template using the @ prefix:

handlebars.register_helper(:list) do |this, context, block|
  "<ul>" + context.each_with_index.map do |x, i|
    if block.keys.include? "data"
      data = handlebars.create_frame(block.data)
      data.index = i
    "<li>" + block.fn(x, data: data) + "</li>"
  end.join + "</ul>"
template = handlebars.compile("{{#list array}}{{@index}}. {{title}}{{/list}}")
template.call(array: [{title: "Memento"}, {title: "Inception"}])
#=> "<ul><li>0. Memento</li><li>1. Inception</li></ul>"

Hash arguments:

When using hash arguments, beware of one gotcha - V8 defines the #hash method for every object. Therefore, to access the hash object of the options argument Handlebars sends to your block, you must use the [] method:

handlebars.register_helper :list do |this, context, options|
  attrs = options[:hash].map{|k,v| "#{k}=\"#{v}\""}.join(' ')
  "<ul #{attrs}>" + context.map{|item| "<li>" + options.fn(item) + "</li>"}.join + "</ul>"
template = handlebars.compile(%({{#list nav id="nav-bar" class="top"}}<a href="{{url}}">{{title}}</a>{{/list}}))
template.call({nav: [{url: 'www.google.com', title: 'Google'}]})
#=> <ul class="top" id="nav-bar"><li><a href="www.google.com">Google</a></li></ul>

Safe Strings

By default, handlebars will escape strings that are returned by your block helpers. To mark a string as safe:

template = handlebars.compile("{{safe}}")
template.call(:safe => proc {Handlebars::SafeString.new("<pre>Totally Safe!<pre>")})


You can directly register partials

handlebars.register_partial("whoami", "I am {{who}}")
handlebars.compile("{{>whoami}}").call(:who => 'Legend') #=> I am Legend

Partials can also be dynamically looked up by defining a partial_missing behavior:

handlebars.partial_missing do |name|
  "unable to find >#{name}"
handlebars.compile("{{>missing}}").call #=> unable to find >missing

Missing partials can also be returned as a function:

count = 0
handlebars.partial_missing do |name|
  lambda do |this, context, options|
    count += 1
    "#{count} miss(es) when trying to look up a partial"
t = handlebars.compile("{{>missing}}")
t.call #=> 1 miss(es) when trying to look up a partial
t.call #=> 2 miss(es) when tyring to look up a partial


In general, you should not trust user-provided templates: a template can call any method (with no arguments) or access any property on any object in the Handlebars::Context.

If you'd like to render user-provided templates, you'd want to make sure you do so in a sanitized Context, e.g. no filesystem access, read-only or no database access, etc.


rspec spec/